I noticed that if I login to SE in a private mode window using Firefox v57.0 Quantum (x64 on Windows 7 Pro) and then open a second private mode window and navigate to e.g. security.stackexchange.com, I see the same login session in that second private browsing window.
On another hand, if I open a third, regular (non-private) window, and navigate to SE, I do not have access to the login session but must login separately.
So what is happening is that all private mode windows have access to the same login sessions. I would have thought that each private mode window would encapsulate its own login sessions.
Is this expected behavior?